This invention belongs to the field of wind propelled vehicles in general and sailboats in particular. It is well known, that increased sail area allows sailboats to reach higher speed. It is also well known, that increased sail area produces higher overturning force. Sailboats are designed to withstand high overturning force allowing to carry large sails and, therefore, to reach high speed. Sailboats have broad hulls and ballast or multiple hulls to achieve high speed. It is also known, that overturning force increases as wind speed increases, while the ability of the sailboat to withstand that force remains largely the same. This leads to practice of reducing sail area, when wind grows stronger, called “reefing the sails”.
Attempts have been made to provide means suitable to counteract overturning force. Thus for example, U.S. Pat. No. 4,703,708 discloses “Steerable Keel” capable of producing counteracting force growing when the speed of the sailboat is rising. In another example, U.S. Pat. No. 4,843,987 discloses “Heel Counteracting Airfoil” capable of producing counteracting force growing when the speed of the wind is rising. However, both examples involve considerable complexity of construction as well as additional resistance to the movement of the sailboat caused by additional airfoil or hydrofoil.
Attempt has been made to provide sails having little or no overturning force. Thus for example, U.S. Pat. No. 5,088,431 discloses two-hull sailboat where sails set at an angle to vertical, specifically top of the sail attached to the mast on the first hull and the bottom of the sail attached to the second hull. For every distance between the hulls, there is the height of the mast such, that the sail creates little overturning force. However, area of that sail is defined and limited by the dimensions of the sailboat.
There is a need for large sail area creating no overturning force.